As there is no way to pass-on the ideas category from Ideas layout to the promoted features layout, I created a custom field to pass on the value. Due to this there are two category fields on the ideas layout which is causing confusion to users.
Currently there is no way to have a hidden field or a read-only field option available while creating custom fields. There are use cases where PMs would like to show info on the portal without users changing it; eg., Release info, roadmap link etc. Having this option for custom fields will help admins to create read-only fields for several purposes both on ideas & features layout.